Air pollution is of concern in many parts of California and is impacted by
both local emissions and also by pollution inflow from the North Pacific
Ocean. In this study, we use the regional chemical transport model WRF-Chem
V3.2 together with the global Model for OZone and Related Chemical Tracers
to examine the CO budget over California. We include model CO tracers for
different emission sources in the models, which allow estimation of the
relative importance of local sources versus pollution inflow on the
distribution of CO at the surface and in the free troposphere. The focus of
our study is on the 15 June–15 July 2008 time period, which coincides
with the aircraft deployment of the NASA Arctic Research of the Composition
of the Troposphere from Aircraft and Satellites (ARCTAS) mission over
California. Model simulations are evaluated using these aircraft
observations as well as satellite retrievals and surface observations of CO.
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Evaluation results show that the model overall predicts the observed CO
fields well, but points towards an underestimate of CO from the fires in
Northern California, which had a strong influence during the study period,
and towards a slight overestimate of CO from pollution inflow and local
anthropogenic sources. The analysis of the CO budget over California reveals
that inflow of CO explains on average 99 &plusmn; 11 ppbV of surface CO during
the study period, compared to 61 &plusmn; 95 ppbV for local anthropogenic direct
emissions of CO and 84 &plusmn; 194 ppbV for fires. In the free troposphere, the
average CO contributions are estimated as 96 &plusmn; 7 ppbV for CO inflow,
8 &plusmn; 9 ppbV for CO from local anthropogenic sources and 18 &plusmn; 13 ppbV
for CO from fires. Accounting for the low bias in the CO fire emission
inventory, the fire impact during the study period might have been up to a
factor 4 higher than the given estimates.